are black, grey, or reddish yellow flies, reminding one by their
appearance of the common house fly or the blue-bottle. They play the
same part in the economy of nature as the sand wasps (p. 129), but
always lay their eggs externally on the skin of the host; the maggots
consequently never prey on those insect larvæ which live in the tissues
of plants or in the soil.
[Illustration:
FIG. 115.—Caterpillar Fly (_Tachina fera_).
]